1. Field
The present application relates to a refresh control circuit and to a memory device including the refresh control circuit.
2. Description of the Related Art
A memory cell of a memory device includes a transistor serving as a switch and a capacitor for storing data in the form of an electrical charge (data). According to whether an electrical charge is stored in the memory cell or a voltage across the capacitor is high or low, the data may be categorized into high (logic 1) and low (logic 0).
Since the data is stored in such a manner that electrical charges are accumulated, no power is consumed in principle. However, the initial charge stored in the capacitor may be degraded due to a leakage current caused by a PN junction of a MOS transistor, and, thus, the data may be lost. In order to prevent such a data loss, the data of the memory cell must be read, and the memory cell must be recharged according to the read information, before the data is lost. Such an operation must be periodically repeated to retain the data.
The operation of recharging the memory cell is commonly referred to as a refresh operation.
FIG. 1 is a diagram illustrating a part of a cell array of a memory device, for explaining a row hammer effect.
Referring to FIG. 1, ‘BL’ and ‘BL+1’ represent bit lines, and ‘WLK−1’, ‘WLK’, and ‘WLK+1’ in the cell array represent three word lines arranged in parallel to each other. In particular, ‘WLK’ with ‘HIGH_ACT’ corresponds to a word line of which the active count or active frequency is high. ‘WLK−1’ and ‘WLK+1’ are word lines arranged adjacent to the word line WLK. Furthermore, ‘CELL_K−1’, ‘CELL_K’, and ‘CELL_K+1’ represent memory cells coupled to the word lines WLK−1, WLK, and WLK+1, respectively. The memory cells CELL_K−1, CELL_K, and CELL_K+1 include cell transistors TR_K−1, TR_K, and TR_K+1 and cell capacitors CAP_K−1, CAP_K, and CAP_K+1, respectively.
When the word line WLK is activated and precharged (deactivated), the voltages of the word lines WLK−1 and WLK+1 rise or fall due to couplings between the word line WLK and the word lines WLK−1 and WLK+1, thereby having an influence on the charges stored in the cell capacitors CAP_K−1 and CAP_K+1. Thus, when the word line WLK is frequently activated and or precharged to toggle between the active state and the precharge state, the charges stored in the capacitors CAP_K−1 and CAP_K+1 may be changed to damage the data stored in the cell capacitors CELL_K−1 and CELL_K+1.
Furthermore, electromagnetic waves generated while the word line WLK toggles between the active state and the precharge state may introduce/discharge electrons into/from the cell capacitors CAP_K+1 and CAP_K+1 included in the memory cells CELL_K−1 and CELL_K+1 coupled to the adjacent word lines WLK−1 and WLK+1, thereby damaging the data of the memory cells. This phenomenon is known in the art as row hammer. Row hammer increases as the memory cell density increases and therefore it is highly desirable to find ways to combat it.